Project Summary Several lines of evidence link infections to the pathogenesis of Alzheimer?s disease (AD). This one-year Supplement application focuses on the potential role of HSV-1 infection. About 70% of older adults are HSV-1 infected. The parent R01 concerns the viral pathogen VZV, shingles, and aging, and is eligible for the Supplement as does not concern Alzheimer?s Disease. The Supplement is related to the parent R01 because HSV-1 has a similar pathogenesis, sequence, and antigenic structure to VZV, and also because aging is a risk factor for both shingles and Alzheimer?s Disease. The Supplement Premise is that the fine specificity of T cells recovered from anatomically and clinically relevant specimens from Mild Cognitive Impairment (MCI) and Alzheimer?s Disease subjects can be used as a biomarker probe possible microbial driver of these disorders. Linkages between HSV-1 and AD include epidemiologic association of AD with HSV-1 seropositivity in APOE4(+) persons, detection of HSV-1 from AD tissues, the ability of HSV-1 to trigger amyloidogenesis in vitro, and interactions between HSV-1 cerebral infection, human APOE genotype, and dementia-like phenotypes in murine models. The Investigative team includes experts in HSV-1 T cell biology, single cell T cell receptor sequencing (scTCRseq) and TCR bioinformatic analysis, advanced histologic of human nervous system tissue, and the clinical study of Mild Cognitive Impairment subjects. The study is home- based at the University of Washington (UW) including the Alzheimer?s Disease Research Center (ADRC), with one domestic and one international collaborative site. Importantly, ongoing NIH grant/subcontract relationships exist between UW and each site and the PI has published multiple recent papers with the Co-Investigators at each site. Aim 1 recovers CSF and matched blood from HSV-1-seropositive persons with MCI and uses wet-lab and TCRseq dry/wet methods to determine if HSV-1-specific T cells are enriched in CSF compared to blood. Aim 2 recovers single T cells from curated AD autopsy tissues from the Netherlands Brain Bank and similarly measures HSV-1 specificity. Together, the Aims leverage the extreme sensitivity and memory characteristics of T cell immunity to examine the hypothesis that HSV-1 is involved in Alzheimer?s Disease